The present invention relates to an exposure head, and more particularly to an exposure head for use in a scanning recording apparatus in which a recording medium is scanned by light beams emitted from a light source and modulated by image information to record a high-quality image free of irregularities on the recording medium.
There is known a scanning recording apparatus in which a recording medium is wound around a recording drum and rotated at a high speed in a main scanning direction whereas an exposure head is moved in an auxiliary scanning direction to record characters, an image, or the like on the recording medium.
The exposure head in the scanning recording apparatus comprises a light-emitting device such as an LED is disposed on a stem. A light beam emitted from the light-emitting device is focused on the recording medium by a lens.
In order to produce a colored image, it is necessary to employ a light beam for emitting light beams to form three primaries (e.g., yellow, magenta, and cyan) on the recording medium. There is an exposure head having light-emitting devices capable of emitting light beams for producing three primaries, the light-emitting devices being arrayed on a single stem. The light beams emitted from the respective light-emitting devices pass through apertures by which flaring light is removed, and are then focused on a recording medium by a lens. The exposure head of this type is advantageous in that it is constructed of fewer parts and more compact than would be if respective light-emitting devices were used as independent exposure heads.
In the above scanning recording apparatus, the recording drum is rotated and at the same time the exposure head is moved to record an image or the like two-dimensionally on the recording medium. Therefore, each of pixels which are formed on the recording medium by the light beams is elongated in the direction of rotation of the recording drum and also in the direction in which the exposure head is moved. Since the speed of rotation of the recording drum in the main scanning direction is higher than the speed of movement of the exposure head in the auxiliary scanning direction, the pixels are elongated in the main scanning direction. The intensity of each of the light beams emitted from the light-emitting devices such as LEDs is of a substantially Gaussian distribution. Therefore, if the output power of each light beam is low and the density of each pixel on the recording medium is low, then the pixel is long in the main scanning direction and narrow in the auxiliary scanning direction. If the light beam output power is high and the pixel density is high, then the pixel is wide in the auxiliary scanning direction. As a result, an image which is produced by the scanning recording apparatus is elongate in the main scanning direction and irregular in the auxiliary scanning direction, resulting in a low image quality.
Where the light-emitting devices are not attached in accurate positions, images focused on the recording medium by the focusing ens are displaced with respect to each other, and a resultant colored image undergoes a color shift and hence has a poor quality. It is not easy to attach the light-emitting devices with high accuracy, and highly accurate attachment of the light-emitting devices is costly.
The light-emitting devices arranged on the stem are protected by a transparent film such as of an acrylic resin. Consequently, a portion of the light beams emitted from the light-emitting devices is reflected by the transparent film and then applied as flaring light to the focusing lens. The produced image may be blurred by the flaring light and lowered in quality. It is possible to cut off a portion of the flaring light with an aperture member disposed between the light-emitting devices and the focusing lens. However, when a portion of the light beam emitted from one light-emitting devices is applied by diffusion or the like to the aperture associated with another light-emitting device, the image is also blurred by flaring light caused by such a light beam.